Tile setting - use of premix grout to level surface?

I have a really small tiling job.

I have a concrete ledge sticking out from a brick wall. Its about 6' long, projects about 9 inches from the wall, and is about one brick high. It is the remains of a fireplace pit.

From left to right it is not level; on the right it is higher by 3/8".

It also slopes downwards a bit from the wall.

I bought a small container of colored pre-mixed adhesive/grout made from limestone & polymer

How thickly can I spread this stuff -- can I use it to level the surface as I set the tile, or do I really have to put on a skim coat of patching cement?

Most pros consider the pre-mixed stuff junk and expensive. It stays soft in the bucket because it needs to dry out to do anything, and can re-emulsify if it gets wet again. It will take a LONG time to set that thick, under a porcelain tile, months. You'd be much better off with some real thinset for this and to set the tile. Note, though, you should do this is a couple of layers since it really is meant to be THINset. An alternative would be to buy a bag of medium set (often called granite and marble) mortar...it has more sand in it and can be installed up to about 1/2" thick (thus, the medium set as opposed to thinset). That can also be used to set whatever tile you want.

The premix was cheaper than a bag & colored too. Guess I'll have to do it the other way.